1. Field of the Invention
The invention refers to a method as well as a device for measuring fluorescence lifetime.
2. Description of the Prior Art
The measurement of fluorescence lifetime is used in various applications. Depending on the material used, the lifetime of fluorescence is a function of external influences such as the pH value, the concentration of oxygen, expansion or temperature. For the purpose of measuring fluorescence lifetime, it is possible to measure the exponential decay of the fluorescence signal after a pulse-shaped excitation. As an alternative, it is possible to periodically excite (for example sinusoidally) the fluorescent medium or material, where the phase shift between the exciting radiation and the fluorescent radiation is measured. This phase shift is a direct measure of the fluorescence lifetime.
It is a problem with the latter measuring method that a phase drift caused by the evaluating electric circuit directly influences the signal to be measured. This drift may be caused, for instance, by external influences such as temperature, but also by the ageing of components. When the measured value changes, it is impossible to tell whether the effect is due to a change in the measured variable (namely the fluorescence lifetime) or to a phase drift in the system.